depending on how much air you are loosing a very small compressor (even a 12V) may be all you need. The little 12V emergency" fill your tire" pumps will not drain your batteries, but most of them are noisy.....I have seen better quality somewhat quieter ones, and have used them in different applications.....FWIW if you remove the plastic housings around the compressor head they run cooler and last longer. Small and inexpensive....must be worth something.

I have a Prevost Liberty that requires lots of air. When Liberty did the conversion they added a 110/220 volt rotary vein compressor, in addition to the normal bus air compressor, that will wake the dead when in a campground. To solve the noise issue I modified two refrigerator compressors that I use when I am parked. They work in parallel and provide plenty of air for the toilet and air bags that the generator sits on. These two compressors while small (and virtually silent) will lift the bus if you give them time. They operate in parallel and are in-line with the main bus air system. I use a series of one way valves to isolate them so that they do not effect the main compressor. If it sounds like something you are interested in I will draw up how I modified the compressors so that they are "dry" and how it is installed. Let me know

Auxiliary air compressors have been the subject of many thread on the various boards.

Two different requirements that I can see. 1) continuous air supply for toilets/doors/etc 2) part time for inflating tires or airing up to leave a campground. A good 120V compressor is great for 1). I suspect there are some OK 12 V units, but generally their volume is pretty low.

For 2), you could consider a York type AC compressor converted to a part time air compressor. I have talked about that in many threads - perhaps the best of those threads is:

As I note in that thread, my goal has been to mount a belt driven hydraulic pump and York compressor on a belt drive from the crank of my generator. I have quite a bit of experience with the York type compressor on my '56 Chevy for the air suspension - works great.

Because that type of compressor is partly cooled by the returning freon, I don't think you can run it for long periods (say 10 minutes or more).

It fits nicely in the engine compartment of my MC9 where the old A/C compressor sat. It runs off the inverter if needed and airs up the bus in about 10 minutes. It is much quieter than the engine and will suffice for air tools in a pinch.

I never use it while running down the road and air brake test confirm the engine compressor is OK.

Dr Steve, the main supplier to the aftermarket/"Hot Rod" market was known as "Air Ride Technologies". They used to have a good site. However, somehow, they became "Ride Tech" (http://www.ridetech.com/) and I their site is not all that user friendly.

I poked around and found that they use Thomas 12V air compressors. The largest I found is:

You can see that the first compressor has a delivery of 1.2 CFM and the Jegs version has a delivery of 0.5 CFM. This is compared to the typical bus engine air compressor - Bendix Tu-Flo 550 which is rated at 13.2 CFM.

You can see that even the best 12V compressors have low CFM ratings compared to 120V units. engine compressors and the York 210 (a bit over 4 CFM).

Quiet air for Bus. I took two refrigerator air compressors and modified them both as follows: 1. With a cut off tool I cut the “ball” around the weld line. 2. Lifting the top I carefully determined the input from the output air line and cut the out put air line close to where it exits the ball on the inside. 3. The unit I was modifying had four small screws holding the compressor motor in place. I took them out and lifted the motor out. 4. I took a good degreaser and washed the motor very well as well as the inside of the compressor ball to remove all shaving caused by cutting the ball in half. While I had the motor out I cut, pinched shut and soldered the capillary vein that is used to pickup oil and refrigerant when the compressor is used in a cooling application (I wanted clean, dry air with no oil being added to my system). 5. I drilled a 3/8 hole in the top half of the “ball and ran the output line out thru the hole. I put sufficient compressor oil in the compressor so that the “splash blade” on the bottom of the compressor motor is in it and splashes oil around to lube the motor. 6. After reinstalling the motor I used a spare Prevost windshield gasket to join the top and bottom of the “ball” . As the compressor is no longer used under pressure inside the ball (originally it was under very high pressure when used in a cooling application), the seal is not critical and only important to keep the oil in as it splashes on the motor to lube it. I modified two compressors like this and hooked them in parallel and then connected them to the aux air tank that Liberty had installed during the conversion.

Coming from the two compressors , I first put the air through a water / oil separator to catch any residual oil or moisture, then run the air through a one way valve and from there thru a “head pressure” relief valve (the output of the head pressure relief valve feeds back to the line from the compressors just after the water / oil separator. This allows the compressors to start each time against 0 head pressure). After that it goes thru an electrical pressure switch that provides power to the compressor anytime the pressure is less than 90 lbs and shuts off at 130lbs. From there it is on to the new 7gallon tank I installed that operates just the toilet. I know this sounds like a lot to doing to have a quiet source of air but it became a challenge (you guys know how that goes). I am sure I could have bought a silent air system cheaper but this really works great and is low maintenance (every now and then I have to check the oil level in the compressors and I change it once a season.).

I know a couple of you guys mentioned that you have to set the regulator on the aux air compressor low enough that it still fills your air bags/air system and yet doesn't trip the air dryer right? (did I say that right?). I have this problem everytime I go to fill the coach air with my pancake compressor...it trips the dryer and then all the air leaks out...no more filling. So what exactly is the PSI number? 90?